At the present, there are in the U.S. alone about 105,000 inadequate bridges. A majority of them are functionally obsolete while a lesser number of them are structurally deficient. The latter are defined as bridges which had to be restricted to light vehicles only or closed, while the former are identified as bridges which can no longer safely service the system of which they are an integral part. The replacement cost for these bridges is in the tens of billions of dollars. A majority of these bridges are intermediate and short span bridges having a length of less than 100 feet. A large part and perhaps again a majority of these bridges have lengths of less than about 50 to 60 feet (hereinafter referred to as "short span bridges").
Applicants have recently invented bridge systems which are ideally suited for building bridges of intermediate and long span lengths at relatively low production and erection costs. These systems are expected to greatly facilitate the replacement of such bridges. Although these systems can also be employed for the construction and erection of short span bridges, some of the cost savings experienced with such bridges are reduced or lost when the bridge span becomes too short, primarily because these longer span bridge systems invented by applicants have a strength and rigidity which exceeds that needed for shorter spans.
Generally speaking, prior art short span bridges forego weight savings experienced by fabricating a variety of plates and extrusions into a steel framework and they instead employ as the main load carrying members a plurality of girders made of steel profiles such as channels, I-beams, wide flange beams and the like which are suspended between bridge support points, normally bridge abutments. The girders are suitably tied together to give the bridge lateral rigidity and a bridge deck is placed on top of the girders. The deck may take a variety of forms and may comprise, for example, wood planking placed tranversely to the length of the bridge and suitably secured, e.g. bolted to the girders, steel deck plates and/or a concrete slab which defines the traffic carrying surface of the bridge.
Although such structures are structurally adequate for the intended purpose, they have a large relatively high deadweight. To a large extent this is a result of a structurally relatively inefficient use of the materials in the girders, especially when formed of extruded profiles. Further this is a result of the fact that such bridges typically use only a few, e.g., 2 or 4 relatively widely spaced apart girders. The distance between the girders must be spanned by the bridge deck and the bridge deck must be sufficiently strong to support loads applied to it between adjacent girders. Yet, such relatively heavy decks do not materially strengthen the bridge in a longitudinal direction and must therefore be considered as dead weight which correspondingly increases the strength requirements placed on the girders.
Thus, high material costs, accentuated by the relatively large weight of such bridges together with the high cost of erecting them render short span bridges constructed in accordance with the prior art relatively expensive. This cost, in turn, limits the rate with which the large number of obsolete bridges can be replaced. Accordingly, there is presently a need for short span bridges which are of a lightweight to minimize material consumption and which can be manufactured and erected at a low cost so as to minimize the cost of short span bridges.